1. Field of the Invention
The present invention relates to fluids mixing and distributing apparatus for vertical downflow reactors. More particularly, the present invention relates to novel reactor internal components which effect improved quench fluid distribution within a vapor-liquid reaction mixture from a first catalyst zone, and a more uniform mixing and redistribution of liquid reactant and vapor reactant prior to their introduction to a second catalyst zone.
2. Description of the Prior Art
Vertical downflow reactors to which the present inventions relate find particular application in a variety of continuous processes wherein vapor-liquid mixture is contacted with a solid catalyst. In many processes, such reactions are exothermic and release substantial amounts of heat. Commonly such reactors for exothermic reactions are designed with a plurality of catalyst zones, each containing solid catalyst, in vertical alignment one with the other. For such exothermic reactions, it is known to provide means between catalyst zones so that a quench fluid at a relatively low temperature may be injected into the reactor vessel to mix with a vapor-liquid reaction mixture from one catalyst zone to absorb excess heat from the reaction mixture before it enters a succeeding catalyst zone.
Such quench fluids may be inert gases, liquids which are unreactive under the reaction conditions, or liquid components of the reaction mixture. The quench fluid may be a vapor component of the reaction mixture and, if so, it replaces a portion of the vapor reactant consumed in the preceding catalyst zone. By means of the quench fluid, a selected ratio of vapor to liquid reactants may be maintained in the reactor vessel as well as controlling the reaction temperature.
Vertical downflow reactors find particular application in processes wherein petroleum oils are reacted in the presence of hydrogen, such as hydrotreating processes for conversion of sulfur and nitrogen components of petroleum oils and hydrocracking processes for conversion of relatively high molecular weight hydrocarbons into lower molecular weight hydrocarbons. Such reactions of petroleum oils with hydrogen are referred to herein as hydrotreating reactions, and such term is intended to include both hydrogen treating for removal of impurities and hydrocracking for reduction of molecular weight. In hydrotreating distillates and heavy oils, such as reduced crudes, residual oils, and vacuum gas oils, vertical down flow reactors having a plurality of catalyst zones are commonly employed. As such hydrotreating reactions are exothermic, it is common practice to inject a hydrogen quench stream into a quench zone between catalyst zones, thereby controlling the reaction temperature within a desired range. Additionally the quench stream maintains the ratio of hydrogen to hydrocarbon at a desirably high value. Heavy oils, when treated at elevated temperatures, have a tendency to thermally crack, forming high molecular weight carbonaceous materials and solid coke. Such thermal cracking is in some degree time dependent. Consequently, it is desirable, within a reactor for treating heavy oils, that no area be available where liquid may stand stagnant for extended time periods at elevated temperatures. Areas of hot, stagnant liquid, in addition to undergoing additional thermal cracking with time, provide areas for carbonaceous deposits to accumulate. Such accumulations of carbonaceous deposits may interfere with flow of reactants through the reactor, and under severe conditions may plug the reactor, preventing all flow therethrough.
The art discloses many constructions for the introduction of a fluid into a reaction mixture for cooling and for effecting thorough mixing of vapor and liquid reaction mixture components to reduce temperature variations across the horizontal cross-section of the reactor, e.g., catalyst bed, through which such reaction mixture flows.
Of particular interest are related U.S. Pat. Nos. 3,824,080 and 3,824,081, both assigned to the assignee of the present application. U.S. Pat. No. 3,824,080 discloses a vertical, downflow reactor having reactor internal parts located intermediate two catalyst zones for intimately mixing vapor and liquid reaction effluent from a first catalyst zone and quench fluid to form a new two-phase vapor-liquid reaction mixture. In addition, reactor internal components are disclosed for evenly distributing the new reaction mixture across the top horizontal cross section of a second catalyst zone.
In this prior art reference, the quench fluid is shown entering a quench zone through a quench nozzle for mixture with the reaction mixture present in the quench zone. No specific details for the quench nozzle construction are given. It is known to provide a quench nozzle construction comprising simply a closed ended conduit having a series of generally downwardly oriented longitudinal exit ports or slots proximate its closed end. The center lines of the slots lie along the centerline of the reactor vessel. An inwardly sloping collector tray, the periphery of which contacts the wall of the reactor vessel below a quench zone, is provided and has an opening concentric with the vertical access with the reactor vessel, for receiving liquid from the quench zone. A mixing means on the upper surface of the collector tray is in axial alignment with and covers the opening in the collector tray and is adapted for mixing and contacting liquid and vapor components from above the collector tray and conveying the mixed vapor and liquid downwardly through the opening in the collector tray. A horizontally disposed dispersing means having a splash plate connected to the collector tray is positioned below and in axial alignment with the collector tray opening for dispersing liquid flowing through the collector tray opening. A liquid distribution means including a perforated distributor tray is attached across the horizontal cross section of the reactor vessel below the collector tray and has a plurality of evenly spaced perforations for the passage of liquid, and a large vapor opening concentric with the vertical axis of the reactor vessel for the passage of vapor. A cylindrical weir surrounds the vapor opening in the perforated tray to prevent liquid flow from the upper surface of the perforated distributor tray through the vapor opening. A second distribution means is attached to the reactor vessel below the liquid distribution means and includes a distributor tray having a plurality of evenly spaced openings with distributor caps covering the distributor tray openings, for evenly distributing vapor and liquid across the horizontal cross section of the reactor vessel second catalyst zone.
U.S. Pat. No. 3,824,081 disclose a substantially similar configuration to that described above further including means to separate any solid particulate contaminants from the reaction mixture prior to the mixture passing into the catalyst zone.
Notwithstanding the significant improvements provided by these devices over the prior art, it has been found that undesirable temperature differentials across the reactor may still occur. As discussed hereinbelow in more detail, the quench fluid exiting the slots in the quench nozzle possesses a horizontal flow component resulting in the quench fluid exiting the slots at an angle and concentrating at the side of the reactor opposite the point at which the quench nozzle enters the reactor vessel. This results in the uneven mixing of quench fluid with the vapor and liquid reaction effluent from the first-catalyst zone and radial temperature variations across the reactor, even when these mixtures pass through the interbed mixing device. While the latter is designed with the aim to thoroughly mix the liquid reactant to eliminate any temperature maldistribution in the reaction mixture entering the second catalyst bed, it has been found that such temperature variations can occur especially at low flow rates, due to incomplete mixing and heat transfer in the mixing device. As one skilled in the art will appreciate, such temperature variations in the fluid as it enters the catalyst bed can have a negative effect on the subsequent reaction as well as catalyst life.
Further examples of interbed quench cooling and interzone mixing devices can be found in U.S. Pat. Nos. 3,433,160; 3,541,000; 3,589,541; 3,746,515; 3,787189; 3,966,420; 4,087252; 4,133,645; 4,138,327; 4,233,269; 4,378,292; 4,481,105; 4,669,890; 4,808,350; and 5,152,967. Some of these devices are complicated; some are prone to plugging; and some need a relatively large space to provide the desired degree of mixing.
It is, therefore, desirable to provide interbed quench cooling and interzone mixing and redistribution components for a reactor vessel which effect improved introduction of a quench fluid into the vapor-liquid reaction mixture from a first catalyst zone and a more thorough mixing and redistribution of the liquid and vapor reactants to be delivered across the top of the horizontal cross-section of a second catalyst zone.